High pressure explosive compositions and method using hollow glass spheres



United States Patent HIGH PRESSURE EE KPLOSIVE COMPOSITIONS AND METHOD USING HOLLOW GLASS SPHERES William C. Thomison and Thomas E. Slykhouse, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Mar. 20, 1967, Ser. No. 624,185 Int. Cl. C06b 19/00; F42d 3/04 US. Cl. 102-23 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION.

It is well known that the density of an explosive af fects its sensitivity. A low' density explosive can usually be detonated with smaller boosters and in a smaller diameter charge than a higher density explosive having the same composition. Slurry explosives having higher densities are particularly difficult to detonate also because of the high heat capacity of the water which is used in most instances as the base of the liquid phase. Typically, density control has been achieved heretofore by the incorporation of voids in the form of air, foamed ammonium nitrate, inert gaseous containing particles such as expanded perlite and other like gaseous containing materials. In other explosive compositions, particles of organic explosives such as, for example, pelletized TNT, gun powder, carbon or light metals such as particulate aluminum have been used as sensitizers. Heretofore, slurry explosive sentially squeezed out of the explosive compositions, thus increasing the density to such a degree that the explosive cannot be readily detonated. The gaseous containing materials which haveheretofore been employed usually collapse under high pressures. Likewise, explosive compositions which employ light metals as sensitizers have failed to detonate under increased pressures such as those-encountered in deep sea siesmic testing and the explosive fracturing of oil wells.

It has been the practice heretofore to use high explosive compositions such as nitroglycerine or slurry explosive compositions containing self-detonating organic explosives, suchas TNT, as sensitizers-when detonation under a high pressure is required. However, it is evident that such explosive compositions are more dangerous to handle than those employing non-explosive sensitizers es pecially in cases where there is an incomplete detonation of the explosive in a well hole. It is therefore preferred to employ-those slurry explosive compositions which do not use conventional self-detonating fuelsbut which employ non-detonating materials, e.g. air and light-metals, as sensitizers. 1 V The patent to Duesing, U.S. Patent No. 2,671,400, teaches the use of air filled globules in specially shaped explosive charges to gain enhanced directional effects of 3,456,589 Patented July 22, 1969 the explosive when detonated. Duesing prefers to employ self-detonating explosives in his shaped charges such as, for example, dynamite and liquid nitroglycerin. The patent does not teach the use of hollow glass particles having particular characteristics which can withstand elevated pressures. The patent also does not teach that unique high pressure characteristics can be achieved when hollow glass particles having the unique characteristics as defined herein are used in slurry explosives to provide and maintain a predetermined density. The novel invention herein is not directed to using entrapped gaseous voids as a sensitizer but is directed to maintaining some reqiured critical densities of a given slurry explosive composition under elevated pressures. The fact that the entrapped gaseous voids also have a sensitizing effect is but one further advantage presented by the present improvement.

It has now been discovered that improved slurry explosive compositions which depend on air andother nonexplosive sensitizers can be made which retain a density required for complete detonation under elevated pressures.

SUMMARY OF THE INVENTION The instant invention comprises an improved method of fracturing underground formations which are under elevated pressures and to improved slurry explosive compositions which retain density requirements under elevated pressures. The invention consists of incorporating into slurry explosive compositions, which usually depend to a certain degree upon gaseous voids, usually air, as a sensitizer, hollow glass particles in suflicient amounts so as to provide a density which allows the most effective propagation of the explosive composition under elevated pressures. The hollow glass particles are characterized by ranging in size from about 10 microns to 250 microns and having a sufficient wall thickness that the particles will not be crushed by the ambient pressure on said explosive prior to detonation. Depending on the pressure under which the explosive is to be detonated, it has been found that particles ranging in size as indicated and having a particle density of from about .2 g./cc. to about .9 g./ cc. have sufficient strength to withstand pressures of at least 1000 psi. It has been found that substantially constant densities can be maintained at pressures up to about 20,000 p.s.i. along with no detrimental loss in sensitivity by incorporating hollow glass particles. with the specific physical characteristics as defined herein .into slurry explosivecompositions, thus providing slurry explosive compositions which can be detonated underelevated pressures without the use of sensitizers which are themselves of a self-detonating material. In the practice of the invention, the improved slurry explosives are placed in boreholes adjacent to an underground formation or preferably are pumped into fissures in aformation, and detonated.

PREFERRED EMBODIMENTS In practice, the improvement of the present invention can be employed with any of the known slurry explosive compositions which depend to a certain degree on gaseous voids as a sensitizer and the components of which will not detrimentally affect the characteristics of the hollow glass particles. The compositions of the invention usually contain at least one inorganic oxidizing salt; a non self-detonating fuel; water and'usually also a gelling or thickening agent.

Examples of salts which are commonly employed in the invention are ammonium nitrate, alkali metal and alkaline earth metal nitrates, perchlorates, and mixtures thereof. Specific examples of typical salts are ammonium nitrate, sodium nitrate, calcium nitrate, sodium perchlorate, potassium perchlorate and the like. Usually, the explosive composition of the present novel composition contains from about 10 percent to about 90 percent by weight of the total composition of the inorganic oxidizer salt; preferably ammonium nitrate and sodium nitrate are employed wherein the ammonium nitrate ranges from about 30 percent to 90 percent by weight of the total composition and the sodium nitrate ranges from about percent to 30 percent by weight of the total composition.

Fuels, which can be employed in the improved explosive composition usually consist of non-explosive carbonaceous and particulate metallic fuels. Non-self-detonating fuels are preferred in the instant improved composition, however, self-detonating fuels such as, for example, organic nitrate and nitro compounds, nitramines and explosive grade nitrocellulose may be employed under certain conditions to provide needed velocity, enhanced sensitivity at low temperature, and the like characteristics.

Examples of carbonaceous non-explosive fuels include finely divided carbon, carbonaceous vegetable products such as corn starch, sugar, wood pulp; organic liquids such as hydrocarbon oils, fatty oils, vegetable oils; urea, crude oils and other liquid hydrocarbons and preferably water-soluble petrolic liquids of low volatility. Generally, up to about 20 percent of the non-explosive carbonaceous fuels are employed in the compositions and preferably from about 5 to 15 percent are employed.

As indicated, particulate metallic fuels also may be employed either alone or in admixture with other fuels. Metallic fuels which can be employed in the novel composition can consist of aluminum, magnesium silicon; alloys of aluminum silicon and magnesium; iron, alloys of iron, ferro-silicon, ferro phosphorous and mixtures and alloys thereof. Particulate metal size generally ranges from about 8 mesh to about 200 mesh and preferably from about 20 to 200 mesh. About 50 percent by weight of metallic fuels can be employed in the present novel invention, however, preferably only from 10 to 30 percent are employed.

As indicated, water-bearing compositions can be employed in the present invention. Usually up to about 25 percent of water may be used, preferably, however, only from about 5 percent to about percent of water is employed and usually at least 8 percent is employed.

Various thickening agents may be employed in the present novel invention and the resultant compositions may vary from pourable solutions, slurries, and dispersions to thick moldable plastic masses. It is preferred that the explosive be maintained in such a condition so as to insure a stable mix having the various components in suspension for substantial periods of time and which are pumpable without the use of excessive pump pressures. Various thickening and gelling agents may be employed. Those commonly used include natural gums such as gum arabic, guar, karaya, synthetic polymers including polyacrylamides and polyvinyl alcohols and the like. Generally from about 0.1 to 10 percent and preferably from about 0.5 to 5 percent by weight of a thickening agent based on the total weight of composition are employed depending on the particular agent chosen and the desired consistency of the explosive.

Generally, it has been found that slurry explosives as defined herein must be maintained at a density of from about 1.0 to about 1.5 g./cc. in order to insure substantially complete propagation upon detonation. The density is usually controlled by the amount of entrapped gaseous voids, usually air, in the explosive mix; heretofore, air has generally been provided in small bubbles in the mixes by agitation of the explosive during the preparation thereof. As indicated, the novel improvement of the present invention consists in admixing into any of a variety of slurry explosive compositions hollow glass particles within well defined proportions and having well defined physical characteristics. As indicated, the density of slurry explosive compositions is important in that when the density is increased above a critical amount, improper propagation of the explosive composition will result. Therefore, the hollow glass particles are added to the particular slurry composition in amounts which will provide a composition having a density which for that particular composition will insure the most efficient detonation under elevated pressures. Generally from about 1 percent to 20 percent by weight of hollow glass particles are used and one, preferably, should not use more hollow glass particles than necessary to provide the explosive composition with a desired density. The most favorable amount of hollow glass particles to be employed is dependent upon the particular slurry employed since the desired operating density varies somewhat accordingly with the composition of the explosive.

The 'wall thickness of the hollow glass particles varies with the pressures under which the explosive compositions are to be employed. A greater wall thickness will be required for high pressures. Hollow glass particles having a particle density of from about 0.2 to 0.9 are usually adequate for use at pressures up to 1000 p.s.i. It is necessary to provide glass particles having sufficient wall thickness so as to not be crushed under the force of the elevated pressures, otherwise the explosive composition will become more dense and consequently less sensitive as the air is squeezed from the explosive.

The hollow glass particles also provide a sensitizing effect upon detonation of the explosive because of the presence of the encapsulated gas in the explosive. It has been found that the size of the gaseous voids within the range disclosed herein does not affect the sensitivity of the explosive composition to any appreciable degree.

The present novel invention is especially adapted for treating underground earth formations to provide a fracturing thereof and particularly at depths where elevated pressures are encountered. In particular, it finds special utility in the treatment of oil and gas wells to fracture and open up passageways away from the borehole in order to increase the production of oil or gas from the well.

Shooting of oil wells can be achieved by placing preformed charges in the borehole; preferably, however, slurry type explosives are pumped into the borehole under pressure so as to enable the explosive to penetrate rock fissures thus obtaining a more complete and effective fracturing of the rock strata to a greater distance away from the borehole. I

Slurry explosives are compressed under the elevated pressures which are encountered at the depths of most oil wells, especially those having a high hydrostatic head. For example, at 1000 feet the pressure under a hydrostatic head is equal to about 500 p.s.i. Heretofore, slurry type explosives which require a certain minimum density for complete propagation could not be fired at this pressure, even when extremely powerful boosters are employed, and even slurry explosives containing self-deton'ating fuels may be ineffective at such pressures.

The present novel method consists in placing a slurry explosive composition containing hollow glass particles in an amount and having characteristics as defined hereinbefore, into the underground earth formation either in preformed charges or preferably pumping the slurry explosive into the bore and into the fissures of the geological formation surrounding theborehole. A small amount of explosive usually is left in the borehole to aid ininitiating the explosive mass. The borehole is usually filled with a liquid, such as, for example, water or cement and the explosive is detonated with a suitable detonation charge. Because of the presence of the hollow'glass particles in the explosive composition, the explosive can be detonated with substantially complete propagation with standard detonation devices. 1

The pressures which are encountered in underground earth formations as, for example, oil wells, may range anywhere from psi. to 20,000 p.s.i. or more dependmg on the depth of the formation. By employing the novel improved method of the present invention the fracturing can be successfully achieved while employing slurryexplosive compositions which are of non self-explosive nature.

The present novel invention is suitable for fracturing underground'earth formations in a variety of areas, including, for example, oil and gas *wells, brine wells, taconite and other like formations.

An especially adaptable explosive composition for the present'novel improvement and which can be employed as the improved method of fracturing underground earth formations is that disclosed in US. Patent No. 3,307,986 and comprises a mixture of alkali metal nitrate to about 60 percent," ammonium nitrate from about to about 85 percent particulate aluminum from about 5 to about 40 percent, water from about 5 to percent, gelling agent from about 0.2 to about 1.5 percent. Thisimprovement comprises adding to the explosive composition hollow glass particles having a particle density ranging from'0.1 g./cc. to 0.9 g./cc. and ranging in size from about 10 microns to about 250 microns and comprising from about 1.0 percent to about. 10 percent by weight of the entire composition depending on the pressure at which the explosive is to be detonated. For complete propagation the density of the explosive composition should be maintained below about 1.4 g./ cc. for percent metal and at 70 F. Lesser metal levels and lower temperatures require reduced densities.

Preferably the composition comprises sodium nitrate from about 5 to about 40 percent; ammonium nitrate from about 15 to about 80 percent; particulate aluminum from about 10 to about 30 percent; water from about 6 to about 16 percent, liquid organic water-miscible liquid extender, from about 2 to about 16 percent; a water swellable gum'and crosslinking agent from about 0.5 to about 1.5 percent, and hollow glass particles having a particle density of from about 0.2 g./cc. to 0.9 g./cc. and ranging in size from about 10 microns to 250 microns and provided in sufiicient quantity so as to maintainthe density of the composition within the range of from about 1.1 g./cc. to about 1.4 g./cc. which usually requires from 2.0 to 10 percent by Weight of hollow'glass particles.

Ordinarily, the aluminum metal employed has a particle size and guage distribution as set forth in Table I.

TABLE I Metal particle size Distribution,

(US. Standard Sieve): percent 30-50 mesh 60-70 50-100 mesh 25-35 100-150 mesh About 5 Metal gauge:

Up to 1 mil 6-8 1 to 5 mils 14-17 5-10 mils 73-77 10-18 mils 15 mils 0.1

Preferably the metal ranges from about 0.025 to about 5 mils in guage. 1

As described in U.S. Patent No. 3,307,986, the term aluminum is meant to include aluminum and aluminum alloys containing at least about 60 percent by weight of aluminum.

Liquid organicfiuid extenders which can be employed consist of monohydroxy alkanols of low molecular weight,

ethylene glycol, propylene glycol, glycerol, formamide, and other like fluid extenders which preferably havea carbon-oxygen ratio such that there is no detrimental competition of the carbon with the metal for available oxygen in the system. Ordinarily, methanol, ethylene glycol, glyc- For example, particularly suitable agents include guar gums, karaya gums, and mixtures thereof and any of a variety of conventional crosslinking agents can be employed.

The desired density can now be achieved and maintained while the explosive is under elevated pressures by the addition of hollow glass particles having the characteristics and in amounts as defined herein'before.

Many different improved explosive compositions may be had by incorporating a suflicient quantity of hollow glass particles having the unique characteristics as set forth herein so as to provide the particular composition at a density which is best suited for complete and ready detonation of the explosives. Generally from about 1 to 10 percent by weight of the unique hollow glass particles are required. Preferably an excess of hollow glass spheres over that required to provide the desired density is not desired since the excess provides unnecessary enhancement of sensitivity, and, in fact, produces less efiicient explosive compositions.

The following examples will facilitate a more thorough understanding of the present novel invention and are included for illustrative purposes only and are not meant to limit the invention to the specific embodiments presented therein.

Example 1 A slurry explosive composition was prepared, the composition of which was as follows:

The explosive composition was prepared by first dispersing the gelling agent into propylene glycol. Water and formamide were mixed and added to the propylene glycol and gelling agent mix. Next the sodium nitrate and ammonium nitrate were added'and mixed. The hollow glass particles were then added to the composition and the entire mixture was warmed to room temperature and the particulate aluminum was added and mixed. Finally, the crosslinking agent was added and the explosive composition was allowed to gell. The hollow glass particles ranged in size from about 10 to 250 microns and had a particle density of about 0.26 g./cc. The final explosive mix had a density of approximately 1.1 g./ cc. The explosive composition was then tested for detonation characteristics under elevated pressures.

The explosive was placed in a 2 inch standard steel pipe having a cover, a welded cap and a pressure tubing connection. The cover was sealed with a standard neoprene O ring. A 37 gram high explosive primer was also placed in the bottom of the pipe. The explosive composition was separated from the cover by a plastic disc to prevent oil from mixing with the explosive prior to detonation.

The pipe was buried in the ground with a small shaped charge next to the booster. The pressure in the pipe was raised to 560 p.s.i., the shaped charge was detonated.

An inspection of the vessel showed it had completely ruptured and the cap was broken thus evidencing the fact that the entire explosive charge had detonated.

Example 2 7 Example 3 An explosive having the following compositions was prepared in a manner similar to that described for preparing the explosive in Example 1.

To this explosive composition was added 1 percent additional expanded polystyrene beads to lower the density of the mix to about 1.3.

The mix was detonated in the same manner as described in Example I and at a pressure of 100 p.s.i. The explosive failed to completely propagate.

A part of the explosive composition described herein without beads was whipped to provide air voids. The mix failed to propagate when pressurized to 100 p.s.i.

Various modifications may be made in the present novel improvement without departing from the spirit or scope thereof for it is understood that we limit ourselves only as defined in the appended claims.

We claim:

1. A method for treating underground earth formations to provide fracturing thereof which comprises:

(a) introducing into a borehole in said formation a inorganic oxidizing salt slurry explosive having entrapped gaseous voids therein, said gaseous voids serving as a sensitizer and a density control thereby rendering said slurry explosive capable of substantially complete propagation when detonated while under an elevated pressure, the entrapped voids in said composition provided by hollow glass particles ranging in size from about 10 microns to about 250 microns and having a sufiicient wall thickness such that said particles are not crushed by the ambient pressure on said explosive;

(b) maintaining an elevated pressure on said explosive prior to detonation;

(c) initiating said explosive thereby fracturing said underground earth formation.

2. The method as defined in claim 1 wherein at least a portion of said explosive penetrates fissures in the earth formation adjacent to said borehole.

3. The method as defined in claim 1 wherein a liquid tamp is provided above said explosive in said borehole.

4. The process as defined in claim 1 wherein said explosive is initiated in a rock formation at least 1000 feet below the surface.

5. The method as defined in claim 1 wherein said hollow glass particles are provided in said slurry explosive in an amount sufiicient to maintain the density of said slurry explosive at from about 1.0 g./cc. to 1.5 g./cc. when said explosive is under an elevated pressure.

6. The method as defined in claim 1 wherein said slurry explosive consists of a water resistant metalized, flowable explosive composition which comprises, on a weight basis to about 60 percent alkali metal nitrate, from about 10 to about 85 percent ammonium nitrate, from about to about 25 percent water, from about 0.2 to about 1.5 percent of a member selected from the group consisting of water swellable gelling and thickening agents, and from about 5 to about 40 percent particulate aluminum, said aluminum ranging in size from about 4 to about 150 mesh U.S. Standard Sieve and having a gauge of from about 0.025 to about 30 mils and being further characterized in having a particle size distribution wherein from about 60 to about 80 percent is from about 12 to about 50 mesh, the balance being substantially 50 to 100 mesh and wherein from about 5 to about percent of the particles have a maximum gauge of about 1 mil, from about 10 to about percent have a gauge of from 1 to about 10 mils, balance ranging from 10 to about 30 mils in gauge; and said explosive composition also containing hollow glass particles dispersed therein, said hollow glass particles ranging in size from about 10 microns to 250 microns and having a particle density ranging from about 0.1 g./cc. to 0.9 g./cc., said hollow glass particles being present in an amount sutficient to provide and maintain a density of less than about 1.4 g./cc.

7. The method as defined in claim 6 wherein said hollow particles are provided in an amount sufficient to provide and maintain the density of said explosive at from about 1.1 g./cc. to about 1.4 g./cc.

8. In an inorganic oxidizing salt slurry explosive compositions capable of substantially complete propagation when detonated while under an elevated pressure, said explosive having entrapped therein gaseous voids as a sensitizer and a density control agent the improvement which comprises:

(a) providing in said explosive composition as a source of entrapped voids, hollow glass particles ranging in size from about 10 microns to about 250 microns and having a sufficient wall thickness so that said particles are not crushed by the ambient pressure on said explosive prior to detonation of said explosive.

9. The improvement as defined in claim 8 wherein said hollow glass particles have a particle density which ranges from about 0.1 g./cc. to 0.9 g./cc.

10. The slurry composition as defined in claim 8 wherein said slurry explosive composition consists of (a) at least one water-soluble inorganic oxidizing salt,

(b) at least one fuel not including self-explosive fuels,

(0) at least 5 percent water, and

(d) hollow glass particles ranging in size from about 10 microns to about 250 microns and having a particle density which ranges from about 0.1 g./cc. to 0.9 g./cc., said particles being present in an amount sufiicient to provide a density of the explosive composition which will insure a substantially complete propagation under elevated pressures.

11. The slurry explosive composition as defined in claim 10 wherein said hollow glass particles are provided in sufficient quantity so as to provide a density of the final explosive composition ranging from about 1.1 g./cc. to about 1.4 g./ cc.

12. The improved slurry explosive as defined in claim 8 wherein said slurry composition comprises on a weight basis to about 60 percent alkali metal nitrate; from about 10 to about percent ammonium nitrate; from about 5 to about 25 percent water; from about 0.2 to about 1.5 percent of a member selected from the group consisting of water swellable gelling and thickening agents, and from about 5 to about 40 percent particulate aluminum, said aluminum ranging in size from about 4 to about 150 mesh US. Standard Sieve and having a gauge of from about 0.025 to about 30 mils and being further characterized in having a particle size distribution wherein from about 60 to about 80 percent is from about 12 to about 50 mesh the balance being substantially 50 to mesh and wherein from about 5 to about 10 percent of the particles have a maximum gauge of about 1 mi], from about 10 to about 85 percent have a gauge of from 1 to about 10 mils, balance ranging from 10 to about 30 mils in gauge, and said explosive composition also containing hollow glass particles dispersed therein, said hollow glass particles ranging in size from about 10 microns to about 250 microns and having a particle density ranging from about 0.1 g./cc. to about 0.9 g./cc. in an amount suffic1ent to provide and maintain an explosive composition having a density of less than 1.4 g./ cc.

13. The improved explosive composition as defined in claim 12 wherein said hollow glass particles are provided in an amount sufficient to provide an explosive composition having a density ranging from about 1.1 g./cc. to about 1.4 g./cc.

14. The improved slurry explosive composition as defined in claim 12 wherein said explosive comprises on a weight basis from about to about 40 percent sodium nitrate; from about 15 to about 80 percent ammonium nitrate; from about 6 to about 16 percent water; from about 2 to about 16 percent of a liquid, organic watermiscible liquid extender; from about 0.5 to about 1.5 percent of a water-swellable, hydrophobic, cross-linkable gum and crosslinking agent therefor; and from about 10 to about percent particulate aluminum, said aluminum having (1) a particle size of from about 30 to about 100 mesh wherein the particle size distribution ranges from about to about percent of about 30 to 50 mesh, from about 25 to about 35 percent of from about 50 to about mesh, balance from about 100 to about mesh and (2) a gauge of from about 1 to about 15 mils and distribution such that about 6 to about 9 percent of the particles have a maximum thickness of about 1 mil, from about 14 to about 17 percent range from about 1 to about 5 mils, from about 73 to about 77 percent range from about 5 to about 10 mils, balance substantially from 10 to about 15 mils, and said explosive composition also containing hollow glass particles dispersed therein, said hollow glass particles ranging in size from about 10 microns to about 250 microns and having a particle density ranging from about 0.1 g./cc. to about 0.9 g./cc. in an amount suflicient to provide and maintain an explosive composition having a density ranging from about 1.1 g./cc. to about 1.4 g./ cc.

15. The method as defined in claim 1 wherein the elevated pressure on said explosive prior to detonation is at least 5 60 psi.

References Cited BENJAMIN R. PADGETT, Primary Examiner U.S. Cl. X.R. 

